Elastic deformation bodies are applied as measuring membranes in pressure sensors. Thus, a pressure sensor usually includes a measuring membrane, which is contactable with the pressure to be measured and experiences a pressure-dependent deformation. The pressure sensor furthermore usually includes a transducer, with which the deformation of the deformation body and measuring membrane, respectively, is converted into a signal, especially into an electrical signal. Examples of such transducers include capacitive transducers, resistive transducers and transducers having a mechanical resonator. An overview of pressure sensors and established transducer principles is given in “Drucksensoren” (“Pressure Sensors”) by Pfeifer and Werthschützky, Berlin 1989.
The development of ceramic pressure sensors, especially those with deformation elements, e.g. measuring membranes, made of Al2O3 ceramic and having a capacitive transducer for industrial process measurements technology, has been moved forward especially by Hegner et al. Noted in this regard by way of example are European patents EP0351701 B1, EP0414871 B1 and EP0445382 B1
Deformation bodies, or measuring membranes, made of a Al2O3 ceramic are advantageous insofar as they exhibit great long-term stability and an absence of hysteresis. Further need for optimization exists, however, as regards breaking stress, because sensitivity and overload resistance, respectively, of a pressure sensor can be increased therewith.
EP 0756586 B1 (Krell) does not directly concern elastic deformation bodies or measuring membranes; yet, it nevertheless contains a good overview of the field of high strength aluminum oxide materials. According to Krell, high strength, highly pure, fine-grained aluminum oxide materials could be achieved only by selection of highly active, fine-grained, high purity, starting powders, with which, by a suitable forming method, a low defect, pore-free structure was achieved. Described by Krell are, without exception, forming methods based on use of a liquid slurry. Through suitable methods such as gel casting or pressure filtration, water is withdrawn from the slurry, and, in the ideal case, a homogeneous green body results. A decisive advantage of liquid forming lies in the fact that, through the initial mobility of the solid particles in the liquid medium, with suitable processing, an as dense and homogeneous a packing of the solid particles as possible is reached. In this way, low-defect structures with high mechanical strengths are achieved in the sinter bodies. Such low-defect structures and, respectively, low-defect, formed bodies cannot, according to Krell, be achieved via usual forming methods, such as, for example, dry pressing. Dry pressing leads to insufficient green density, and, respectively, to density inhomogeneities in the formed bodies, and, in the sintering, to hardness-decreasing and strength-decreasing defects. All customary press methods are based on the densification of granulates produced via spray drying, among other things. As a function of the character of the granular material, it cannot be avoided that cavities form between and in the densified granulates. These cavities cannot be completely removed during the sinter process. Mechanical properties are negatively influenced by these defects remaining in the structure.
The concepts for the forming methods of Krell, as based on a slurry, are thus so sketched. However, these forming methods are very complex and too expensive for mass production of deformation elements for pressure sensors.
Additionally, Krell permits comparatively large proportions of glass-forming, sinter aids, especially SiO2. These sinter aids enable a reduced sinter temperature, and thereby lessen grain growth, which can lead to increased fracture stress as well as hardness. On the other hand, a glass phase is an impediment for application of the sinter body as an elastic body in measurements technology. Investigations in connection with the present invention have shown, namely, that glass phases in the structure lessen the relaxation capability of the sinter body and its resistance to corrosion. Limiting grain size via SiO2 is infeasible from a practical point of view.
It is, consequently, an object of the present invention to provide a method, with which elastic bodies made of Al2O3 ceramic, especially elastic deformation bodies, especially measuring membranes for pressure sensors, are manufacturable at acceptable costs in mass production.